Purpose: Botulinum toxin (BTX) injections are used in cosmetic surgery to efface facial wrinkles. Botulinum toxin relaxes the muscle by preventing the release of the neurotransmitter acetylcholine at the neuromuscular junction located at the posterior muscle surface causing local muscle paralysis. The purpose of this study is to provide anatomic knowledge of muscle belly depths of the frontalis, corrugator supercilii, procerus, and orbicularis oculi muscles in an attempt to improve the efficacy of BTX treatment of glabellar, forehead, and lateral eyelid rhytides. Methods: Six-millimeter punch biopsies were obtained from 7 fresh cadavers. Biopsies were taken from the corrugator supercilii, frontalis, procerus, and orbicularis oculi muscles at the sites of routine BTX injection. Specimens were fixed in formalin, and representative H&E-stained sections were used to measure muscle surface depths by light microscopy equipped with digital camera that includes a digital micrometer. One-way analysis of variance test analyses were used to identify statistical differences between measured muscle depths. Results: The measured anterior muscle depth of the corrugator supercilii, frontalis, procerus, and orbicularis oculi was found to be 4.2 ± 0.6, 3.9 ± 0.6, 2.9 ± 0.4, and 2.3 ± 0.7 mm, respectively. The anterior muscle surface of the corrugator supercilii and frontalis was found to be deeper than that of the procerus and orbicularis oculi ( P < .001). The posterior surface depth of the corrugator supercilii, frontalis, procerus, and orbicularis oculi was found to be 6.6 ± 0.8, 5.1 ± 0.7, 4.9 ± 0.7, and 3.8 ± 1.0 mm, respectively. The posterior surface depth of the corrugator supercilii was found to be significantly deeper than that of the frontalis, procerus, and orbicularis oculi ( P < .001); the posterior surface depth of the frontalis and procerus was deeper than that of the orbicularis oculi ( P < .001). The muscle belly width of the corrugator supercilii, frontalis, procerus, and orbicularis oculi measured 2.5 ± 0.9, 1.1 ± 0.4, 2.0 ± 0.6, and 1.5 ± 0.5 mm, respectively. The corrugator supercilii was found to be thicker than the frontalis and orbicularis oculi, while the procerus was found to be thicker than the frontalis ( P < .001). Conclusion: The findings above demonstrate statistical differences in the posterior muscle surface depth of the corrugator supercilii, frontalis, procerus, and orbicularis oculi which can be used clinically to improve BTX injection efficacy when used to efface facial rhytides.
